Method and apparatus for transmitting reference signal in multiple antenna system

ABSTRACT

A method and an apparatus for transmitting a reference signal in a multiple antenna system are provided. The method includes transmitting a first reference signal based on a first sequence through a first antenna group, and transmitting a second reference signal based on a second sequence through a second antenna group, wherein the first reference signal and the second reference signal are transmitted through a same radio resource.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and an apparatus for transmitting a referencesignal in a multiple antenna system.

BACKGROUND ART

In next generation multimedia mobile communication systems, which havebeen actively studied in recent years, there is a demand for a systemcapable of processing and transmitting a variety of information (e.g.,video and radio data) in addition to the early-stage voice service. 3rdgeneration wireless communication is followed by a 4th generationwireless communication which is currently being developed aiming atsupport of a high-speed data service of 1 gigabits per second (Gbps) indownlink and 500 megabits per second (Mbps) in uplink. Wirelesscommunication systems are designed for the purpose of providing reliablecommunication to a plurality of users irrespective of users' locationsand mobility. However, a wireless channel has an abnormal characteristicsuch as path loss, noise, fading due to multipath, an inter-symbolinterference (ISI), the Doppler effect due to mobility of a userequipment, etc. Therefore, various techniques have been developed toovercome the abnormal characteristic of the wireless channel and toincrease reliability of wireless communication.

Orthogonal Frequency Division Multiplexing (OFDM), Multiple InputMultiple Output (MIMO), etc., are techniques for supporting reliablehigh-speed data services.

An OFDM system capable of reducing an inter-symbol interference effectwith a low complexity is taken into consideration as one of post-3rdgeneration wireless communication systems. In the OFDM, a serially inputdata symbol is converted into N parallel data symbols (where N is anatural number), and is then transmitted by being carried on N separatesubcarriers. The subcarriers maintain orthogonality in a frequencydimension. In a mobile communication market, a standard is expected tobe changed from a conventional code division multiple access (CDMA)system to an OFDM-based system.

The MIMO technique improves data transmission/reception efficiency byusing multiple transmit (Tx) antennas and multiple receive (Rx)antennas. Examples of the MIMO technique include spatial multiplexing,transmit diversity, beamforming, etc. A MIMO channel matrix depending onthe number of Rx antennas and the number of Tx antennas can bedecomposed into a plurality of independent channels. Each independentchannel is referred to as a layer or a stream. The number of layers isreferred to as a rank.

For the purpose of data transmission/reception, system synchronizationacquisition, channel information feedback, etc., there is a need toestimate an uplink channel or a downlink channel in a wirelesscommunication system. Channel estimation is a process of recovering a Txsignal by compensating for signal distortion in an environment where arapid change occurs due to fading. In general, channel estimationrequires a reference signal known to both a transmitter and a receiver.The reference signal is also referred to as a pilot.

In the OFDM system, reference signals may be allocated by using twomethods, i.e., a first method in which the reference signals areallocated to all subcarriers and a second method in which the referencesignals are allocated between data subcarriers. The first method uses asignal (e.g., a preamble signal) consisting of only reference signals.The first method has a significantly improved channel estimationperformance in comparison with the second method, but has a decreaseddata transmission rate. Therefore, the second method can be used toincrease the data transmission rate. The second method may result indeterioration of the channel estimation performance since density of thereference signals is decreased. Therefore, it is required that thereference signals are properly arranged to minimize the deterioration ofthe channel estimation performance.

When the transmitter transmits a reference signal p and the receiverreceives an Rx signal y, the Rx signal y can be expressed by thefollowing equation.

MathFigure 1

y=h·p+n  [Math.1]

Herein, h denotes a channel on which the reference signal istransmitted, and n denotes thermal noise generated in the receiver.

In this case, the reference signal p is known to the receiver. Thereceiver can estimate the channel by using the reference signal p. Theestimated channel h′ can be expressed by the following equation.

MathFigure 2

h′=y/p=h+n/p=h+n′  [Math.2]

Accuracy of the estimated channel h′ is determined according to n′. Forthe accuracy of the estimated channel h′, n′ has to converge to zero.Channel estimation may be performed by using a large number of referencesignals to minimize an influence of n′. The receiver can compensate forthe estimated channel to recover data transmitted by the transmitter.

Since antennas of a multiple antenna system respectively correspond todifferent channels, each antenna has to be considered in the designingof a reference signal structure. In a multiple antenna system, it iseffective to use even power transmission in which each antenna has thesame Tx power as much as possible. Even power transmission usingmultiple antennas can result in decrease in implementation cost andimprovement in system performance.

However, when the reference signal structure is designed so that evenpower transmission is possible in the multiple antenna system, areference signal overhead may be significantly increased. The referencesignal overhead can be defined as a ratio of the number of subcarriersfor transmitting the reference signal to the number of all subcarriers.When the reference signal overhead is great, there is a problem in thatthe number of data subcarriers for transmitting data in practice isdecreased. This results in decrease in a data processing load anddeterioration in spectrum efficiency. As a result, an overall systemperformance may deteriorate.

Accordingly, there is a need for a method and an apparatus foreffectively transmitting a reference signal in a multiple antennasystem.

DISCLOSURE OF INVENTION Technical Problem

The present invention provides a method and an apparatus fortransmitting a reference signal in a multiple antenna system.

Technical Solution

In an aspect, a method for transmitting a reference signal in a multipleantenna system is provided. The method includes transmitting a firstreference signal based on a first sequence through a first antenna groupand transmitting a second reference signal based on a second sequencethrough a second antenna group, wherein the first reference signal andthe second reference signal are transmitted through a same radioresource.

Preferably, the radio resource comprises an orthogonal frequencydivision multiplexing (OFDM) symbol and a subcarrier.

Preferably, the second sequence is obtained by cyclic shifting the firstsequence in a time domain.

Preferably, the second sequence is obtained by multiplying the firstsequence by a phase shift in a frequency domain.

Preferably, the second sequence is orthogonal to the first sequence.

In another aspect, a method for transmitting a reference signal in amultiple antenna system using M antennas (M≧2, where M is a naturalnumber) is provided. The method includes transmitting a first referencesignal based on a first sequence through a first antenna group andtransmitting a second reference signal based on a second sequencethrough a second antenna group, wherein the M antennas are paired intotwo antenna groups, the first antenna group is one antenna group of thetwo paired antenna groups, and the second antenna group is the otherantenna group of the two paired antenna groups, and wherein the firstreference signal and the second reference signal are transmitted througha same radio resource.

Preferably, a resource block is defined per each of the M antennas, theresource block comprises a plurality of OFDM symbols and a plurality ofsubcarriers, each element of the resource block is a resource element,and the radio resource is a resource element at a specific position inthe resource block.

Preferably, the first reference signal and the second reference signalare transmitted through the same number of radio resources in one OFDMsymbol within the resource block for each of the M antennas.

In still another aspect, a transmitter is provided. The transmitterincludes a first antenna group, a second antenna group and a referencesignal generator generating a first reference signal based on a firstsequence to be transmitted through the first antenna group, andgenerating a second reference signal based on a second sequence to betransmitted through the second antenna group, wherein the firstreference signal and the second reference signal are transmitted througha same radio resource.

In still another aspect, a receiver is provided. The receiver includes aradio frequency (RF) unit transmitting and/or receiving a radio signaland a processor coupled with the RF unit and configured to receive afirst reference signal based on a first sequence and receive a secondreference signal based on a second sequence, wherein the first referencesignal and the second reference signal are received through a same radioresource.

Preferably, the radio resource comprises an OFDM symbol and asubcarrier.

Preferably, the second sequence is obtained by cyclic shifting the firstsequence in a time domain.

Preferably, the first reference signal is a reference signal for a firsttransmit antenna, and the second reference signal is a reference signalfor a second transmit antenna, and the processor configures to estimatea channel of the first transmit antenna in accordance with the firstreference signal, and estimate a channel of the second transmit antennain accordance with the second reference signal.

Advantageous Effects

A method and an apparatus for effectively transmitting a referencesignal in a multiple antenna system are provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a wireless communication system.

FIG. 2 is a block diagram showing a radio protocol architecture for auser plane.

FIG. 3 is a block diagram showing a radio protocol architecture for acontrol plane.

FIG. 4 shows mapping between a downlink logical channel and a downlinktransport channel.

FIG. 5 shows mapping between a downlink transport channel and a downlinkphysical channel.

FIG. 6 shows a structure of a radio frame.

FIG. 7 shows an example of a resource grid for one downlink slot.

FIG. 8 shows a structure of a subframe.

FIG. 9 shows an example of a reference signal structure when a basestation uses one antenna.

FIG. 10 shows an example of a reference signal structure in whichshifting is performed in a frequency domain when a base station uses oneantenna.

FIG. 11 shows an example of a reference signal structure when a basestation uses two antennas.

FIG. 12 shows an example of a reference signal structure when a basestation uses four antennas.

FIG. 13 is a graph showing an example of power allocation for eachresource element in an OFDM symbol.

FIG. 14 is a graph showing another example of power allocation for eachresource element in an OFDM symbol.

FIG. 15 is a flow diagram showing an example of a method fortransmitting a reference signal in a multiple antenna system.

FIG. 16 shows an example of a reference signal structure.

FIG. 17 shows an example of a cyclically shifted reference signalstructure.

FIG. 18 shows a first example of a reference signal structure using codedivision multiplexing (CDM).

FIG. 19 shows a second example of a reference signal structure usingCDM.

FIG. 20 shows a third example of a reference signal structure using CDM.

FIG. 21 shows a fourth example of a reference signal structure usingCDM.

FIG. 22 shows a fifth example of a reference signal structure using CDM.

FIG. 23 is a block diagram showing an example of a transmitter usingmultiple antennas.

FIG. 24 is a block diagram showing an apparatus for a wirelesscommunication.

MODE FOR THE INVENTION

FIG. 1 is a block diagram showing a wireless communication system. Thismay be a network structure of a 3rd generation partnership project(3GPP) long term evolution (LTE)/LTE-advanced (LTE-A). The LTE may bealso referred to as an evolved-universal mobile telecommunicationssystem (E-UMTS). The wireless communication system can be widelydeployed to provide a variety of communication services, such as voices,packet data, etc.

Referring to FIG. 1, an evolved-UMTS terrestrial radio access network(E-UTRAN) includes at least one base station (BS) 20 providing a userplane and a control plane towards a user equipment (UE) 10.

The UE 10 may be fixed or mobile, and may be referred to as anotherterminology, such as a mobile station (MS), a user terminal (UT), asubscriber station (SS), a mobile terminal (MT), a wireless device, etc.The BS 20 may be a fixed station that communicates with the UE 10 andmay be referred to as another terminology, such as an evolved node-B(eNB), a base transceiver system (BTS), an access point, etc. There areone or more cells within the coverage of the BS 20. Interfaces fortransmitting user traffic or control traffic may be used between the BSs20. The BSs 20 are interconnected with each other by means of an X2interface. The BSs 20 are also connected by means of an S1 interface toan evolved packet core (EPC), more specifically, to the mobilitymanagement entity (MME) by means of the S1-MME and to the servinggateway (S-GW) 30 by means of the S1-U. The S1 interface supports amany-to-many relation between the BS 20 and the MME/S-GW 30.

Hereinafter, downlink means communication from the BS 20 to the UE 10,and uplink means communication from the UE 10 to the BS 20. In downlink,a transmitter may be a part of the BS 20 and a receiver may be a part ofthe UE 10. In uplink, a transmitter may be a part of the UE 20 and areceiver may be a part of the BS 20.

The UE belongs to one cell. The cell to which the UE belongs is referredto as a serving cell. The BS which provides the serving cell with acommunication service is referred to as a serving BS. The wirelesscommunication system is a cellular system in which another cell isadjacent to the serving cell. The adjacent another cell is referred toas a neighbor cell.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The first layer is a physical (PHY) layer. The second layer canbe divided into a medium access control (MAC) layer, a radio linkcontrol (RLC) layer, and a packet data convergence protocol (PDCP)layer. The third layer is a radio resource control (RRC) layer.

FIG. 2 is a block diagram showing a radio protocol architecture for auser plane. FIG. 3 is a block diagram showing a radio protocolarchitecture for a control plane. They illustrate the architecture of aradio interface protocol between the UE and the E-UTRAN. The user planeis a protocol stack for user data transmission. The control plane is aprotocol stack for control signal transmission.

Referring to FIGS. 2 and 3, between different PHY layers (i.e., a PHYlayer of a transmitter and a PHY layer of a receiver), information iscarried through a physical channel. The PHY layer is coupled with a MAClayer, i.e., an upper layer of the PHY layer, through a transportchannel. Data is transferred between the MAC layer and the PHY layerthrough the transport channel. The PHY layer provides the MAC layer andupper layers with information transfer services through the transportchannel.

The MAC layer provides services to an RLC layer, i.e., an upper layer ofthe MAC layer, through a logical channel. The RLC layer supportsreliable data transmission. The PDCP layer performs a header compressionfunction to reduce a header size of an Internet protocol (IP) packet.

An RRC layer is defined only in the control plane. The RRC layercontrols radio resources between the UE and the network. For this, inthe RRC layer, RRC messages are exchanged between the UE and thenetwork. The RRC layer serves to control the logical channel, thetransport channel, and the physical channel in association withconfiguration, reconfiguration and release of radio bearers (RBs). An RBmeans a logical path provided by a first layer (i.e. PHY layer) andsecond layers (i.e. MAC layer, RLC layer and PDCP layer) for datatransmission between the UE and the network. Configuring the RB includesdefining radio protocol layers and characteristics of channels toprovide a service and defining specific parameters and operationschemes. The RB may be classified into a signaling RB (SRB) and a dataRB (DRB). The SRB is used as the path to transfer RRC messages in thecontrol plane and the DRB is used as the path to transfer user data inthe user plane. When an RRC connection is established between an RRClayer of the UE and an RRC layer of the network, it is called that theUE is in an RRC connected mode. When the RRC connection is notestablished yet, it is called that the UE is in an RRC idle mode.

A non-access stratum (NAS) layer belongs to an upper layer of the RRClayer and serves to perform session management, mobility management, orthe like.

FIG. 4 shows mapping between a downlink logical channel and a downlinktransport channel. The section 6.1.3.2 of 3GPP TS 36.300 V8.3.0(2007-12) Technical Specification Group Radio Access Network; EvolvedUniversal Terrestrial Radio Access (E-UTRA) and Evolved UniversalTerrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2(Release 8) may be incorporated herein by reference.

Referring to FIG. 4, a paging control channel (PCCH) is mapped to apaging channel (PCH). A broadcast control channel (BCCH) is mapped to abroadcast channel (BCH) or a downlink shared channel (DL-SCH). A commoncontrol channel (CCCH), a dedicated control channel (DCCH), a dedicatedtraffic channel (DTCH), a multicast control channel (MCCH), and amulticast traffic channel (MTCH) are mapped to the DL-SCH. The MCCH andMTCH are also mapped to a multicast channel (MCH).

A type of each logical channel is defined according to a type ofinformation to be transmitted. The logical channel is classified into acontrol channel and a traffic channel.

The control channel is used to transmit control plane information. TheBCCH is a downlink channel for broadcasting system control information.The PCCH is a downlink channel for transmitting paging information andis used when a network does not know a location of a UE. The CCCH is achannel for transmitting control information between the UE and thenetwork and is used when there is no RRC connection established betweenthe UE and the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting multimedia broadcast multicast service(MBMS) control information. The MCCH is used by UEs that receive anMBMS. The DCCH is a point-to-point bi-directional channel fortransmitting dedicated control information between the UE and thenetwork, and is used by UEs having an RRC connection.

The traffic channel is used to transmit user plane information. The DTCHis a point-to-point channel for transmitting user information and existsin both uplink and downlink. The MTCH is a point-to-multipoint downlinkchannel for transmitting traffic data and is used by UEs that receive anMBMS.

The transport channels are classified by how and with whatcharacteristics data are transferred over the radio interface. The BCHis broadcast in the entire coverage area of the cell and has a fixed,pre-defined transport format. The DL-SCH is characterized by support forhybrid automatic repeat request (HARM), support for dynamic linkadaptation by varying modulation, coding, and transmit (Tx) power,possibility to be broadcast in the entire cell, and possibility to usebeamforming, support for both dynamic and semi-static resourceassignment, support for UE discontinuous reception (DRX) to enable UEpower saving, and support for MBMS transmission. The PCH ischaracterized by support for DRX to enable UE power saving andrequirement to be broadcast in the entire coverage area of the cell. TheMCH is characterized by support for requirement to be broadcast in theentire coverage area of the cell and support for an MBMS singlefrequency network (MBSFN).

FIG. 5 shows mapping between a downlink transport channel and a downlinkphysical channel. The section 5.3.1 of 3GPP TS 36.300 V8.3.0 (2007-12)may be incorporated herein by reference.

Referring to FIG. 5, a BCH is mapped to a physical broadcast channel(PBCH). An MCH is mapped to a physical multicast channel (PMCH). A PCHand a DL-SCH are mapped to a physical downlink shared channel (PDSCH).The PBCH carries a BCH transport block. The PMCH carries the MCH. ThePDSCH carries the DL-SCH and the PCH.

Several downlink physical control channels are used in a PHY layer. Aphysical downlink control channel (PDCCH) informs a UE of resourceassignment of the PCH and DL-SCH, and also informs the UE of HARQinformation related to the DL-SCH. The PDCCH may carry an uplinkscheduling grant which informs the UE of resource assignment for uplinktransmission. A physical control format indicator channel (PCFICH)informs the UE of the number of orthogonal frequency divisionmultiplexing (OFDM) symbols used for transmission of the PDCCHs within asubframe. The PCFICH is transmitted in every subframe. A physical hybridARQ indicator channel (PHICH) carries HARQ acknowledgement(ACK)/negative-acknowledgement (NACK) in response to uplinktransmission.

FIG. 6 shows a structure of a radio frame.

Referring to FIG. 6, the radio frame consists of 10 subframes. Onesubframe consists of two slots. Slots included in the radio frame arenumbered with slot numbers 0 to 19. A time required to transmit onesubframe is defined as a transmission time interval (TTI). The TTI maybe a scheduling unit for data transmission. For example, one radio framemay have a length of 10 milliseconds (ms), one subframe may have alength of 1 ms, and one slot may have a length of 0.5 ms.

The structure of the radio frame is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe may change variously.

FIG. 7 shows an example of a resource grid for one downlink slot.

Referring to FIG. 7, the downlink slot includes a plurality of OFDMsymbols in a time domain and N^(DL) resource blocks (RBs) in a frequencydomain. The OFDM symbol is for representing one symbol period. The OFDMsymbol may also be referred to as an orthogonal frequency divisionmultiple access (OFDMA) symbol, single carrier-frequency divisionmultiple access (SC-FDMA) symbol, etc. in accordance with multipleaccess scheme. The number N^(DL) of resource blocks included in thedownlink slot depends on a downlink transmission bandwidth configured ina cell. For example, in a 3GPP LTE system, N^(DL) may be any one valuein the range of 60 to 110. One RB includes a plurality of subcarriers inthe frequency domain.

Each element on the resource grid is referred to as a resource element(RE). The resource element on the resource grid can be identified by anindex pair (k, l) within the slot. Herein, k (k=0, . . . , N^(DL)×12−1)denotes a subcarrier index in the frequency domain, and l (l=0, . . . ,6) denotes an OFDM symbol index in the time domain.

Although it is described herein that one RB includes 7×12 resourceelements consisting of 7 OFDM symbols in the time domain and 12subcarriers in the frequency domain for example, the number of OFDMsymbols and the number of subcarriers in the RB are not limited thereto.Thus, the number of OFDM symbols and the number of subcarriers maychange variously depending on a cyclic prefix (CP) length, a frequencyspacing, etc. For example, when using a normal CP, the number of OFDMsymbols is 7, and when using an extended CP, the number of OFDM symbolsis 6. In one OFDM symbol, the number of subcarriers may be selected from128, 256, 512, 1024, 1536, and 2048. The structure of an uplink slot maybe same as that of the downlink slot.

FIG. 8 shows a structure of a subframe.

Referring to FIG. 8, the subframe includes two consecutive slots. Amaximum of three OFDM symbols located in a front portion of a 1st slotwithin the subframe correspond to a control region. The remaining OFDMsymbols correspond to a data region. Control channels such as a PCFICH,a PHICH, a PDCCH etc., can be assigned to the control region. A PDSCHcan be assigned to the data region. A UE can read data informationtransmitted through the PDSCH by decoding control informationtransmitted through the PDCCH. Although the control region includesthree OFDM symbols herein, this is for exemplary purposes only. Thenumber of OFDM symbols included in the control region of the subframecan be known by using the PCFICH.

FIG. 9 shows an example of a reference signal (RS) structure when a BSuses one antenna.

Referring to FIG. 9, R0 denotes a resource element used to transmit areference signal through an antenna 0. In one OFDM symbol, R0s arelocated with a spacing of 6 subcarriers. The number of R0s is constantwithin a resource block.

Hereinafter, a resource element used to transmit a reference signal isreferred to as a reference symbol. Resource elements other than thereference symbol can be used for data transmission. A resource elementused for data transmission is referred to as a data symbol. Onereference signal is transmitted for each antenna. A reference signal foreach antenna is transmitted through reference symbols.

When a serving cell and a neighbor cell use the same-structuredreference signal, collision may occur between the cells. To avoid thecollision, a reference signal can be protected by shifting referencesymbols in a frequency domain on a subcarrier basis, or by shifting thereference symbols in a time domain on an OFDM symbol basis.

FIG. 10 shows an example of an RS structure in which shifting isperformed in a frequency domain when a BS uses one antenna.

Referring to FIG. 10, a 1st cell uses reference symbols located with aspacing of 6 subcarriers in one OFDM symbol. Thus, by shifting thereference symbols on a subcarrier basis in the frequency domain, atleast 5 neighbor cells (2nd to 6th cells) can use reference symbolsrespectively located in different resource elements.

Accordingly, collision of reference signals is inevitable among the 1stto 6th cells. For example, if vshift denotes a variable indicating thenumber of subcarriers for shifting reference symbols in the frequencydomain, vshift can be expressed by the following equation.

MathFigure 3

v _(shift) =N _(cell) _(—) _(ID) mod 6  [Math.3]

Herein, N_(cell) _(—) _(ID) denotes a cell identifier (ID).

A reference signal may be multiplied by a predetermined reference signalsequence when transmitted. For example, the reference signal sequencemay be generated based on a pseudo-random (PN) sequence, an m-sequence,etc. The reference signal sequence may be generated based on a binarysequence or a complex sequence. When the BS transmits the referencesignal multiplied by the reference signal sequence, interference of areference signal received from a neighbor cell can be reduced and thuschannel estimation performance can be improved in a UE. The referencesignal sequence may be used on an OFDM symbol basis in one subframe. Thereference signal sequence may vary according to a cell ID, a slot numberin one radio frame, an OFDM symbol index in a slot, a CP length, etc.

Referring to FIG. 9, in an OFDM symbol including reference symbols, thenumber of reference symbols for each antenna is 2. Since a subframeincludes N^(DL) resource blocks in the frequency domain, the number ofreference symbols for each antenna is 2×N^(DL) in one OFDM symbol. Thus,a reference signal sequence has a length of 2×N^(DL).

When r(m) denotes a reference signal sequence, the following equationshows an example of a complex sequence used as r(m).

MathFigure 4

$\begin{matrix}{{r(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2\; m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2\; m} + 1} \right)}}} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

Herein, m is 0, 1, . . . , 2N^(max,DL)−1. N^(max,DL) denotes the numberof resource blocks corresponding to a maximum bandwidth. For example, inthe 3GPP LTE system, N^(max,DL) is 110. c(i) is a PN sequence and can bedefined by a Gold sequence having a length of 31. The following equationshows an example of a sequence c(i) having a length of 2×N^(max,DL).

MathFigure 5

C(n)=(x ₁(n+N _(c))+x ₂(n+N _(c)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+ ³)+x ₂(n+2)+x ₁(n+1)+x ₁(n))mod 2  [Math.5]

Herein, N_(C) is 1600, x₁(i) denotes a 1st m-sequence, and x₂(i) denotesa 2nd m-sequence. For example, the 1st m-sequence can be initializedwith x₁(0)=1, x₁(n)=0 (n=1, 2, . . . , 30) at the start of each OFDMsymbol. The 2nd m-sequence can be initialized according to a cell ID, aslot number in one radio frame, an OFDM symbol index in a slot, a CPlength, etc. at the start of each OFDM symbol.

When a system has a bandwidth smaller than N^(max,DL), a certain portionof a reference signal sequence generated to have a length of2×N^(max,DL) can be selected to be used.

In multi-antenna transmission, a BS uses a plurality of antennas, thereis one resource grid defined per antenna. In FIG. 11 and FIG. 12described below, reference symbols for all antennas are shown on thesame resource grid for convenience of explanation. Rp denotes a resourceelement used to transmit a reference signal through an antenna p (where,pε{0, 1, 2, 3}). Rp is not used for any transmission on any otherantennas except for the antenna p. This is to avoid interference betweenantennas.

FIG. 11 shows an example of an RS structure when a BS uses two antennas.

Referring to FIG. 11, R0 and R1 do not overlap with each other. In oneOFDM symbol, each Rp is located with a spacing of 6 subcarriers. In asubframe, the number of R0s is equal to the number of R1s. In addition,in one OFDM symbol, the number of R0s is identical to the number of R1s.

FIG. 12 shows an example of an RS structure when a BS uses fourantennas.

Referring to FIG. 12, R0 to R3 do not overlap with one another. In oneOFDM symbol, each Rp is located with a spacing of 6 subcarriers. In asubframe, the number of R0s is equal to the number of R1s, and thenumber of R2s is equal to the number of R3s. In the subframe, the numberof R2s and R3s is less than the number of R0s and R1s.

R0 and R1 are paired with each other, and R2 and R3 are paired with eachother. The paired R0 and R1 may be referred to as a first pair, and thepaired R2 and R3 may be referred to as a second pair. The first pair andthe second pair are included in different OFDM symbols. Therefore, areference signal of an antenna 0 and a reference signal of an antenna 1are paired with each other and are then transmitted in the same OFDMsymbol. For example, in both of a 1st slot and a 2nd slot within asubframe, the reference signal of the antenna 0 and the reference signalof the antenna 1 are paired and transmitted in OFDM symbols whose OFDMsymbol indices are 0 and 4 (l=0, 4). Further, reference signals of anantenna 2 and an antenna 3 are paired and transmitted on the same OFDMsymbol. For example, in both of the 1st slot and the 2nd slot within thesubframe, the reference signal of the antenna 2 and the reference signalof the antenna 3 are paired and transmitted in an OFDM symbol whose OFDMsymbol index is 1 (l=1).

FIG. 13 is a graph showing an example of power allocation for eachresource element in an OFDM symbol. The index of the OFDM symbol is 0(l=0). The x axis represents an antenna index, the y axis represents asubcarrier index within a resource block, and the z axis representspower.

Referring to FIG. 13, antennas 0 and 1 each has two reference symbolsand four data symbols. Antennas 2 and 3 each has four data symbols. Ifthe index of the OFDM symbol is 4, the same graph can be applied bychanging the antenna index.

FIG. 14 is a graph showing another example of power allocation for eachresource element in an OFDM symbol. The index of the OFDM symbol is 1(l=1). The x axis represents an antenna index, the y axis represents asubcarrier index within a resource block, and the z axis representspower.

Referring to FIG. 14, antennas 0 and 1 each has four data symbols.Antennas 2 and 3 each has two reference symbols and four data symbols.

As such, In an OFDM symbol whose OFDM symbol index is 0 or 4 (l=0, 4),paired reference signals of the antennas 0 and 1 (a first pair) aretransmitted, and a large amount of power is allocated to the antennas 0and 1 each. In an OFDM symbol whose OFDM symbol index is 1 (l=1), pairedreference signals of the antennas 2 and 3 (a second pair) aretransmitted, and a large amount of power is allocated to the antennas 2and 3 each.

That is, the first pair and the second pair are transmitted in differentOFDM symbols. Therefore, when power boosting is performed on referencesymbols, power is boosted only for antennas with specific pairing in oneOFDM symbol. Further, it is difficult to achieve even power transmissionfor each antenna. Even power transmission allows each antenna of amultiple antenna system to have the same Tx power as much as possible.Even power transmission using multiple antennas can result in decreasein implementation cost and improvement in system performance. To enableeven power transmission using the multiple antennas, it is preferablethat each antenna has the same number of reference symbols in one OFDMsymbol.

Within one subframe, the number of paired R0s and R1s is double of thenumber of paired R2s and R3s in a time domain. The antennas 2 and 3 eachhas a lower channel estimation performance than that of the antennas 0and 1 each in a time selective channel. Whether a channel is the timeselective channel can be known by based on a coherence time. Thecoherent time is inversely proportional to a Doppler spread. When usinga multiple antenna scheme which is designed to achieve even powertransmission for each antenna, system performance may significantlydeteriorate if channel estimation performance is uneven for eachantenna. Therefore, there is a need for a method for transmitting areference signal to provide even power transmission for each antenna inthe multiple antenna system.

When using the multiple antenna system, data can be recovered only whena reference signal for each antenna is identifiable. To avoidinterference between reference signals for respective antennas,frequency division multiplexing (FDM), time division multiplexing (TDM),or code division multiplexing (CDM) can be used. In the FDM, a referencesignal for each antenna is transmitted by being divided in a frequencydomain. In the TDM, the reference signal for each antenna is transmittedby being divided in a time domain. In the CDM, the reference signal foreach antenna is transmitted by using a different sequence. When the FDDand TDM are used to transmit reference signals through multipleantennas, reference symbols for each antenna do not overlap with oneanother. When the CDM is used, resource elements used for transmissionof a reference signal for each antenna may overlap with one another.Therefore, when the CDM is used, the reference signals can betransmitted through the multiple antennas without significantlyincreasing a reference signal overhead.

Hereinafter, a method for transmitting a reference signal to achieveeven power transmission for each antenna in a multiple antenna system byusing CDM will be described.

FIG. 15 is a flow diagram showing an example of a method fortransmitting a reference signal in a multiple antenna system.

Referring to FIG. 15, a BS transmits a 1st reference signal (RS1)through a 1st antenna group and transmits a 2nd reference signal (RS2)through a 2nd antenna group (step S110). The RS1 and the RS 2 aretransmitted through a same radio resource. The RS1 generates based on a1st sequence, and the RS2 generates based on a 2nd sequence. A UEestimates a channel in accordance with the RS1 and the RS2 (step S120).

As such, by using the CDM, a resource element used as a reference symbolof one antenna can be multiplexed with a reference symbol of anotherantenna. The RS1 for one antenna may generate based on the 1st sequence,and the RS2 for another antenna may generate based on the 2nd sequenceorthogonal to the 1st sequence. When the 1st sequence and the 2ndsequence are orthogonal to each other, the UE can recover the 1stsequence and the 2nd sequence without interference. In addition, the 2ndsequence may use a sequence having a low correlation with the 1stsequence.

For example, when using four antennas, the CDM is applied by using the2nd sequence so that reference signals of the antennas 2 and 3 arepaired in an OFDM symbol in which reference signals of the antennas 0and 1 using the 1st sequence are paired. In doing so, each OFDM symbolincluding reference symbols can simultaneously transmit referencesignals of all antennas. Since the number of reference symbols of eachof all antennas is identical in one OFDM symbol, even power transmissionis possible.

If the 1st sequence and the 2nd sequence are orthogonal to each other,any sequence can be used as the 1st sequence and the 2nd sequence. Ingeneral, a reference signal sequence is a random sequence. Hereinafter,the 1st sequence is referred to as a pseudo-random (PN) sequence. Forexample, the 2nd sequence may be obtained by cyclic shifting or delayingthe PN sequence in a time domain. When cyclic shifting is performed inthe time domain, the 2nd sequence is configured in a form in which thePN sequence is multiplied by a phase shift sequence in a frequencydomain. The phase shift sequence is hereinafter referred to as anorthogonal sequence (OS).

The 1st sequence may be the reference signal sequence of Equation 4. Ifri(m) denotes the 2nd sequence obtained by cyclic shifting the referencesignal sequence r(m), r_(i)(m) can be expressed by the followingequation.

MathFigure 6

r _(i)(m)=r(m)e ^(−jθ)1^(m)  [Math.6]

Herein, i=1, 2, . . . , N (where N is a natural number). N 2nd sequencescan be generated in accordance with i. N may differ in accordance with achannel condition. θ_(i) is a cyclic shift value, and can be expressedby the following equation for example.

MathFigure 7

$\begin{matrix}{\theta_{i} = \frac{2{\pi \left( {i - 1} \right)}}{N}} & \left\lbrack {{Math}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

If θ_(i) is 0, the 2nd sequence is identical to the 1st sequence. Thatis, a reference signal sequence used for a reference signal is atwo-layered sequence which is obtained by multiplying the PN sequence bythe OS. Hereinafter, ‘PN+OS’ denotes a reference signal sequenceobtained by multiplying the PN sequence by the OS. For example,‘PN0+OS0’ and ‘PN0+OS1’ are obtained by applying different cyclicshifting to the same PN sequence, and are orthogonal to each other. Thatis, orthogonal reference signal sequences can be generated bymultiplying the same PN sequence by different OSs. The PN sequence maychange in accordance with an OFDM symbol position. The reference signalsequence may be obtained by cyclic shifting a changed PN sequence.

The cyclic shift value θ_(i) has a sufficient interval to identify animpulse response of a channel for each antenna. It is assumed that awireless communication system operates in a channel environment where aneffective OFDM symbol length is 66.7 μs, and a maximum delay spread is 5μs. In this case, a cyclic shift value is provided at least every 5 μs.Therefore, 12 cyclic shifts can be identified. If reference symbols arelocated with a spacing of 6 subcarriers in one OFDM symbol, the numberof available cyclic shifts is decreased by 6-fold. That is, the numberof cyclic shift values may be 12/6=2.

For convenience of explanation, terminologies are defined as follows.Hereinafter, Px denotes a position of a resource element to which areference signal is mapped. When a reference signal of one antenna ismapped to a resource element P1, the reference signal for the antenna ismapped to all resource elements P1. Rp denotes a reference symbol usedfor transmission of a reference signal of an antenna p. Rp correspondsto a reference signal sequence obtained by multiplying a PN sequence byan OS (i.e., PN+OS).

FIG. 16 shows an example of an RS structure.

Referring to FIG. 16, P1 and P2 are paired in one OFDM symbol, and P3and P4 are paired in another OFDM symbol. Px (x=0, 1, 2, 3), Rp (p=0, 1,2, 3), and a reference signal sequence corresponding to Rp can beconfigured in various forms.

The following table shows a first example of a position of a resourceelement Px, a reference symbol Rp for each antenna, and a referencesignal sequence.

TABLE 1 R0 R1 R2 R3 P1 PN0 + OS0 — — — P2 — PN0 + OS0 — — P3 — — PN0 +OS0 — P4 — — — PN0 + OS0

In P1, a reference signal of an antenna 0 is transmitted. In P2, areference signal of an antenna 1 is transmitted. In P3, a referencesignal of an antenna 2 is transmitted. In P4, a reference signal of anantenna 3 is transmitted. The CDM is not used herein. This is the sameformat as the RS structure of FIG. 12 in which four antennas are used.

The following table shows a second example of a position of a resourceelement Px, a reference symbol Rp for each antenna, and a referencesignal sequence.

TABLE 2 R0 R1 R2 R3 P1 PN0 + OS0 — PN0 + OS1 — P2 — PN1 + OS0 — PN1 +OS1 P3 PN0 + OS0 — PN0 + OS1 — P4 — PN1 + OS0 — PN1 + OS1

Each antenna may use a different reference signal sequence. In an OFDMsymbol in which P1 and P2 (or P3 and P4) are paired, different PNsequences may be used in P1 and P2 (or P3 and P4). In P1 and P3 each, R0and R2 are multiplexed using orthogonal reference signal sequences. R0and R2 use the same PN sequence, and use different OSs. In P2 and P4each, R1 and R3 are multiplexed using orthogonal reference signalsequences. R1 and R3 use the same PN sequence, and use different OSs.Therefore, each antenna has the same number of reference symbols in theOFDM symbol in which P1 and P2 (or P3 and P4) are paired. The FDM andCDM are used herein. In this case, even power transmission for eachantenna is possible.

The example of Table 2 may change variously as long as satisfying therequirement of even power transmission for each antenna. That is, theFDM and CDM are used for even power transmission, and irrespective of anantenna number, each antenna has the same number of reference symbols inthe OFDM symbol in which P1 and P2 (or P3 and P4) are paired.

The following table shows a third example of a position of a resourceelement Px, a reference symbol Rp for each antenna, and a referencesignal sequence.

TABLE 3 R0 R1 R2 R3 P1 — PN0 + OS0 — PN0 + OS1 P2 PN0 + OS1 — PN0 + OS0— P3 — PN0 + OS0 — PN0 + OS1 P4 PN0 + OS1 — PN0 + OS0 —

In an OFDM symbol in which P1 and P2 (or P3 and P4) are paired, the samePN sequence may be used in P1 and P2 (or P3 and P4). In P1 and P3 each,R1 and R3 are multiplexed using orthogonal reference signal sequences.R1 and R3 use the same PN sequence, and use different OSs. In P2 and P4each, R0 and R2 are multiplexed using orthogonal reference signalsequences. R0 and R2 use the same PN sequence, and use different OSs.Therefore, each antenna has the same number of reference symbols in theOFDM symbol in which P1 and P2 (or P3 and P4) are paired.

For example, OS1 used by R3 in P1 may have a phase of π/2 correspondingto a cyclic shift. In this case, the cyclic shift can be determined bydifferently setting a start phase offset for each OFDM symbol.

FIG. 17 shows an example of a cyclically shifted RS structure.

Referring to FIG. 17, a cyclic shift value θ_(i) within one OFDM symbolis π/2. If an OFDM symbol index is 0 (l=0), a start phase offset of P1is 0. If the OFDM symbol index is 4 (l=4), the start phase offset of P1is π/4.

The following table shows a fourth example of a position of a resourceelement Px, a reference symbol Rp for each antenna, and a referencesignal sequence.

TABLE 4 R0 R1 R2 R3 P1 PN0 + OS0 — PN0 + OS1 — P2 — PN1 + OS0 — PN1 +OS1 P3 PN2 + OS0 — PN2 + OS1 — P4 — PN3 + OS0 — PN3 + OS1

In an OFDM symbol in which P1 and P2 (or P3 and P4) are paired, the samePN sequence may be used in P1 and P2 (or P3 and P4). In P1 and P3 each,R0 and R2 are multiplexed using orthogonal reference signal sequences.In P2 and P4 each, R1 and R3 are multiplexed using orthogonal referencesignal sequences. Therefore, each antenna has the same number ofreference symbols in the OFDM symbol in which P1 and P2 (or P3 and P4)are paired. Even if a reference signal is transmitted through oneantenna, a different PN sequence may be used when a numeral x of Pxdiffers due to a different OFDM symbol position. For example, for areference signal for antenna 0, PN0 is used in P1 and PN2 is used in P3.In this case, a cell-based sequence can be randomized in a multi-cellenvironment.

When reference signals of multiple antennas are multiplexed using theCDM, power used before multiplexing can be evenly distributed to beused. However, if a receiver cannot support an RS structure using theCDM, power of the reference signal is halved and thus channel estimationperformance may significantly decrease. Accordingly, it is need tocontrol power of each antenna's reference signal multiplexed using theCDM. A 1st sequence multiplexed with a 2nd sequence on the same resourceelement may have a different power ratio. The 1st sequence is a PNsequence, and the 2nd sequence is obtained by cyclic shifting the 1stsequence in a time domain. For example, power of a specific antenna maybe controlled so that a specific antenna has a robust channel estimationperformance. In addition, reference signal power of an antenna using the2nd sequence obtained by cyclic shifting the 1st sequence may be set toa relatively small value.

A signal received in P1 can be expressed by the following equation.

MathFigure 8

{tilde over (r)} _(P1)(m)=h ₁ √{square root over (α)}·r(m)e ^(−j0) +h₂√{square root over (1−α)}·r(m)e ^(−jθ)1^(m)  [Math.8]

Herein, m is 0, 1, . . . , 2N^(max,DL)−1. h₁ and h₂ each denotes achannel, and a denotes a power control factor. If total Tx power is 1, αsatisfies 0≦α≦1. For example, if α=1, it implies that a reference symbolfor one antenna is mapped for each Px as shown in Table 1. If α=0.5, the1st and 2nd sequences multiplexed using the CDM have the same power. Byregulating α, power of a reference signal sequence for each antenna canbe regulated.

Although an RS structure when using 4 antennas has been described up tonow for example, a method for transmitting a reference signal by usingthe CDM can also apply when four or more antennas are used.

In a method described below, a reference signal is transmitted using theCDM when 8 antennas are used. Hereinafter, for convenience ofexplanation, the 1st sequence is referred to as a pseudo-random sequence1 (PN1), and the 2nd sequence is referred to as a pseudo-random sequence2 (PN2). As described above, the PN2 may be obtained by cyclic shiftingor delaying the PN1 in a time domain. When cyclic shifting is performedin the time domain, the PN2 is configured in a form in which the PN1 ismultiplied by a phase shift sequence in a frequency domain. Rp denotes areference symbol of an antenna p. A numeral p is any one value selectedfrom 0, 1, 2, . . . , 7.

In FIG. 18 to FIG. 21 described below, R0 and R4 are multiplexed on P1.R1 and R5 are multiplexed on P2. R2 and R6 are multiplexed on P3. R3 andR7 are multiplexed on P4. R0, R1, R2, and R3 each use the PN1 as thereference signal sequence. R4, R5, R6, and R7 each use the PN2 as thereference signal sequence. This can be expressed by the following table.

TABLE 5 R0 R1 R2 R3 R4 R5 R6 R7 P1 PN1 PN2 P2 PN1 PN2 P3 PN1 PN2 P4 PN1PN2

As such, the PN1 and the PN2, which are orthogonal to each other, can beused to transmit reference signals of 8 antennas.

FIG. 18 shows a first example of an RS structure using CDM.

Referring to FIG. 18, an RS structure using 4 antennas (see FIG. 12) isextended to an RS structure using 8 antennas. Channel estimationperformances of antennas 4 to 7 are respectively identical to those ofantennas 0 to 3. A reference symbol overhead is 14% similarly to the RSstructure using 4 antennas (see FIG. 12). In an OFDM symbol includingreference symbols, the number of data symbols is properly maintained.Thus, power boosting of the reference symbols can be easily performed.In particular, a data symbol structure in a control region is notchanged. Therefore, the structure of FIG. 18 can be compatible with the3GPP LTE system.

However, the number of reference symbols used in antennas 0, 1, 4, and 5is double of that used in antennas 2, 3, 6, and 7, respectively.Therefore, each of the antennas 2, 3, 6, and 7 has poorer channelestimation performance than each of the antennas 0, 1, 4, and 5. Inparticular, each of the antennas 2, 3, 6, and 7 may have inferiorchannel estimation performance in a time selective channel.

FIG. 19 shows a second example of an RS structure using CDM.

Referring to FIG. 19, antennas 0 to 7 each has the same number ofreference symbols. Thus, each of the antennas 0 to 7 has the samechannel estimation performance. A reference symbol overhead is 19%. Inaddition, reference symbols of all antennas are transmitted in one OFDMsymbol. Therefore, even power transmission is possible among theantennas 0 to 7.

However, the number of data symbols is small in an OFDM symbol includingreference symbols. Therefore, power boosting of reference symbols islimited. In particular, when the power boosting of the reference symbolsis limited in a control region, reliability of a control channel maydeteriorate.

FIG. 20 shows a third example of an RS structure using CDM.

Referring to FIG. 20, the CDM is not used in a region used as a controlregion, and only reference symbols for 4 antennas are transmitted on theregion. On the remaining regions other than the control region,reference symbols for 8 antennas may be transmitted using the CDM. Thecontrol region and an RS structure in the control region can bemaintained identical to those in the 3GPP LTE system, therebymaintaining compatibility with the 3GPP LTE system.

FIG. 21 shows a fourth example of an RS structure using CDM. In this RSstructure, a reference symbol overhead is decreased to 14% whilemaintaining a characteristic of the RS structure of FIG. 19.

Referring to FIG. 21, antennas 0 to 7 each has the same number ofreference symbols. Thus, the antennas 0 to 7 each has the same channelestimation performance. The reference symbol overhead is 14%. Inaddition, in one OFDM symbol, reference symbols of all antennas aretransmitted. Therefore, even power transmission is possible among theantennas 0 to 7. Further, the number of data symbols is properlymaintained for an OFDM symbol including reference symbols. Therefore,power boosting of the reference signals can be easily performed.

FIG. 22 shows a fifth example of an RS structure using CDM.

Referring to FIG. 22, R0 and R4 are multiplexed on P1. R1 and R5 aremultiplexed on P2. R2 and R6 are multiplexed on P3. R3 and R7 aremultiplexed on P4. In this case, R0, R1, R2, and R3 each use PN1 as areference signal sequence, and R4, R5, R6, and R7 each use PN2 as thereference signal sequence. R0 using PN1 and R2 using PN2 are multiplexedon P5. R1 using PN1 and R3 using PN2 are multiplexed on P6. R2 using PN1and R0 using PN2 are multiplexed on P7. R3 using PN1 and R1 using PN2are multiplexed on P8. This can be expressed by the following table.

TABLE 6 R0 R1 R2 R3 R4 R5 R6 R7 P1 PN1 PN2 P2 PN1 PN2 P3 PN1 PN2 P4 PN1PN2 P5 PN1 PN2 P6 PN1 PN2 P7 PN2 PN1 P8 PN2 PN1

As shown in the above table, instead of using the same sequence for allreference symbols of the same antenna, different sequences can be usedaccording to a position of an OFDM symbol.

FIG. 23 is a block diagram showing an example of a transmitter usingmultiple antennas. The transmitter may be a part of a BS or a part of aUE.

Referring to FIG. 23, a transmitter 100 includes a reference signalgenerator 110, a data processor 120, and a MIMO processor 130.

The reference signal generator 110 generates a reference signal asdescribed up to now. The data processor 120 generates a data symbol byperforming data processing. For example, data processing includeschannel coding, modulation, etc. The MIMO processor 130 processes a datasymbol and a reference signal according to a MIMO scheme depending on Txantennas 190-1, . . . , 190-Nt. The data symbol and the reference signalare mapped to resource elements for each of the Tx antennas 190-1, . . ., 190-Nt, and then an OFDM symbol is generated. The generated OFDMsignal is transmitted on each of the Tx antennas 190-1, . . . , 190-Nt.

FIG. 24 is a block diagram showing an apparatus for a wirelesscommunication. The apparatus may be a part of a UE. An apparatus 50includes a processor 51, a memory 52, a radio frequency (RF) unit 53, adisplay unit 54, and a user interface unit 55. The processor 51 may beconfigured to implement proposed functions, procedures and/or methodsdescribed in this description. Layers of the radio interface protocolmay be implemented in the processor 51. The processor 51 may provide thecontrol plane and the user plane. The function of each layer can beimplemented in the processor 51. The memory 52 is operatively coupledwith the processor 51 and stores a variety of information to operate theprocessor 51 (e.g., an operating system, applications, and generalfiles). The display unit 54 displays a variety of information of theapparatus 50 and may use a well-known element such as a liquid crystaldisplay (LCD), an organic light emitting diode (OLED), etc. The userinterface unit 55 can be configured with a combination of well-knownuser interfaces such as a keypad, a touch screen, etc. The RF unit 53 isoperatively coupled with the processor 51 and transmits and/or receivesradio signals.

The processor 51 may include application-specific integrated circuit(ASIC), other chipset, logic circuit and/or data processing device. Thememories 52 may include read-only memory (ROM), random access memory(RAM), flash memory, memory card, storage medium and/or other storagedevice. The RF units 53 may include baseband circuitry to process radiofrequency signals. When the embodiments are implemented in software, thetechniques described herein can be implemented with modules (e.g.,procedures, functions, and so on) that perform the functions describedherein. The modules can be stored in memories 52 and executed byprocessor 51. The memories 52 can be implemented within the processor 51or external to the processor 51 in which case those can becommunicatively coupled to the processor 51 via various means as isknown in the art.

Although the reference signal structure has been described up to now fordownlink communication, it can also apply for uplink communication.

As described above, a method and an apparatus for effectivelytransmitting a reference signal in a multiple antenna system can beprovided. By the use of a reference signal structure using CDM,different antennas can transmit respective reference signals by usingthe same resource element. That is, the number of reference symbols foreach antenna can be increased without increasing a reference signaloverhead. Therefore, the reference signal structure can be designed sothat even power transmission for each antenna is possible. In doing so,implementation cost can be decreased and system performance can beimproved.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

1. A method for transmitting a reference signal in a multiple antennasystem, the method comprising: transmitting a first reference signal fora first antenna using a first sequence through a first antenna group;and transmitting a second reference signal for a first antenna using asecond sequence, wherein a first portion of the first reference signalis cyclic-shifted from a second portion of the first reference signal,the first portion of the first reference signal and the second portionof the first reference signal are transmitted through a same timeresource, and the first reference signal and the second reference signalare transmitted through a same radio resource.
 2. The method of claim 1,wherein the first reference signal and the second reference signal aretransmitted through at least one orthogonal frequency divisionmultiplexing (OFDM) symbol.
 3. The method of claim 1, wherein a value ofa phase shift between the first portion of the first reference signaland the second portion of the first reference signal is determined basedon a radio resource in which the first portion of the first referencesignal is allocated.
 4. (canceled)
 5. The method of claim 1, wherein thesecond sequence is orthogonal to the first sequence. 6-13. (canceled)14. The method of claim 1, wherein the first sequence has a lowcorrelation with the second sequence.
 15. The method of claim 1, whereinthe first reference signal and the second reference signal are used forrecovering data.
 16. The method of claim 1, wherein the method isperformed by a base station.
 17. A method for receiving a referencesignal by a user equipment, the method comprising: receiving a firstreference signal which is transmitted by a first antenna of atransmitter using a first sequence; and receiving a second referencesignal which is transmitted by a second antenna of the transmitter usinga second sequence, wherein a first portion of the first reference signalis cyclic-shifted from a second portion of the first reference signal,the first portion of the first reference signal and the second portionof the first reference signal are transmitted through a same timeresource, and the first reference signal and the second reference signalare transmitted through a same radio resource.